[reaction: see text] The base-promoted cyclization of optically pure N-(p-methoxybenzyl)-N-(2-chloro)propionyl amino acid derivatives resulted in a diastereo- and enantioselective approach to valuable 1,3,4,4-tetrasubstituted beta-lactams. The stereochemical outcome of the reaction is exclusively governed by the configuration of the N-(2-chloro)propionyl moiety.
The addition of 3,4-dimethoxybenzyl thiol 8, as a benzyl thiol surrogate, to racemic 4-hydroxycyclopent-2-enone 2 and 4-hydroxycyclohex-2-enone 15 gave the corresponding cis-adducts (+/-)-3-(3,4-dimethoxybenzylthio)-4-hydroxycyclopentanone 4b and (+/-)-3-(3,4-dimethoxybenzylthio)-4-hydroxycyclohexanone 16 with good diastereocontrol. In both cases, subsequent treatment with vinyl acetate, in the presence of a lipase enabled enantiomer resolution. Thus, (+)-16 and the acetate of its enantiomer, (-)-(1R,2S)-2-(3,4-dimethoxybenzylthio)-4-oxocyclohexyl acetate, (-)-17 were isolated in 98% enantiomeric excess. Based on the 1,4-dioxygenation pattern, (-)-17 can be used to prepare both enantiomers of 4-(tert-butyldimethylsilyloxy)cyclohex-2-enone 19. Firstly, saponification, with a sub-stoichiometric amount of NaOMe, followed by a one-pot silyl ether formation-sulfide elimination sequence gave (+)-19. Then using the same starting material a 6-step sequence, featuring a diastereoselective NaBH4 reduction and a Cope-type sulfoxide elimination, gave (-)-19.
Alkaloids U 0600Rapid Synthesis of the Tetrahydroquinoline Alkaloids: Angustureine (XI), Cuspareine (XIII) and Galipinine (XV). -A one-pot three-step method for the synthesis of 1,2-disubstituted tetrahydroquinolines involves the C-2 addition of organolithium reagents, N-alkylation and subsequent hydrogenation of the intermediate dihydroquinoline derivatives. It is successfully applied to the preparation of the title alkaloids including an unoptimized asymmetric version for (XV). -(O'BYRNE, A.; EVANS*, P.; Tetrahedron 64 (2008) 35, 8067-8072; Cent. Synth. Chem. Biol., Univ. Coll., Belfield, Dublin, Ire.; Eng.) -Mischke 50-200 M e
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